There are many different parameters which one may wish to measure in a well, some associated with the general well environment, and others relating to particular stages in the completion and production of the well, and even to particular procedures carrying out in the well.
Particular instances where it is desired to measure conditions include production testing of wells, which is a well established practice to understand which zones are in production and what they are producing from a well. Another example is the monitoring of changes in the internal diameter of a flow path in an oil well casing which may be subject to reduction in diameter through deposition of scale, or through formation collapse, or to an increase in diameter caused by corrosion or mechanical damage.
Other instances of well sensing occur when it is desired to monitor the performance of a particular tool or part of a tool. For example, during gas lift of a well (where gas is used to help lift hydrocarbons from reservoir to surface), gas is injected under pressure from the surface into the production tubing annulus. Down the length of the production tubing are located gas lift valves. Each are set to a pre defined cracking pressure, so that they meter gas into the production tubing, which in turn helps to lift the oil to surface. If a valve is not working correctly or is not allowing sufficient gas to enter the production tubing, then production is not optimized and the net flow rate is not maximized.
Conventional tools used to perform these measurements typically require electrical power; for example, in measuring the flow rate, a flow diverter is used to direct the flow to the central area of the production tubing where a turbine flow meter is used to determine the combined flow at that point in the well.
It will be appreciated that any sensors and also require associated electronics, power supplies and associated hardware has to tolerate the harsh chemical, temperature and pressures subjected to at depth in an oil or gas well.
A common type of communications link includes a wireline in which one or more electrical conductors route power and data between a downhole component and the surface equipment. Other conveyance structures can also carry electrical conductors to enable power and data communications between a downhole component and surface equipment. To communicate over an electrical conductor, a downhole component typically includes electrical circuitry and sometimes power sources such as batteries. Such electrical circuitry and power sources are prone to failure for extended periods of time in the typically harsh environment (high temperature and pressure) that is present in a wellbore.
Another issue associated with running electrical conductors in a wireline, or other type of conveyance structure, is that in many cases the wireline extends a an intervention, remedial, or investigative tool into a wellbore. Conventionally, such intervention, remedial, or investigative tools are carried by a wireline, slickline, coiled tubing, or some other type of conveyance structure. If communication is desired between the intervention, remedial, or investigative tool and the surface equipment, electrical conductors are run through the conveyance structure. As noted above, electrical conductors are associated with various issues that may prove impractical in some applications.